Method of optimal data transmission for improving data transmission rate in multi-hop wireless network

ABSTRACT

There is provided a method for optimal data transmission for improving a data transmission rate of a node with variable transmission power in a multi-hop wireless network, the method including the steps of: obtaining channel state information about a current wireless channel of the node; calculating a carrier sensing range in the number of hops using the obtained channel state information, a target signal-to-interference ratio, and a contention window size in order to minimize data collision; calculating the number of nodes attempting data transmission based on signals received from neighbor nodes, the number of the nodes attempting data transmission being the number of contention nodes; and setting transmission power adaptively according to the calculated carrier sensing range value and the contention node numbers and transmitting data with the set of transmission power.

TECHNICAL FIELD

The present invention relates to a method of optimal data transmissionfor improving a data transmission rate in a multi-hop wireless network;and, more particularly, to a method of optimal data transmission forimproving a data transmission rate in a multi-hop wireless network,which can minimize a data collision and maximize an end-to-endthroughput by adaptively calculating a carrier sensing range value for anode with variable transmission power to control the transmission poweraccording to the calculated carrier sensing range value and byadaptively adjusting a carrier sensing threshold value for a node withconstant transmission power.

BACKGROUND ART

There is no prior patent technology related to a method of modifying aphysical carrier sensing range in an ad-hoc network. Some related papersare published in the journals and Proceedings of IEEE CommunicationSociety, but such papers also have little relation with the presentinvention.

In IEEE 802.11 communication standards called Local Area Network (LAN),each terminal (i.e., a node) and an access point (AP) might use the samefrequency band. In that case, the terminal (node) and the access point(AP) can recognize each other as one network member to communicate dataand control packets with each other.

There are two modes in the IEEE 802.11 standards. One is aninfrastructured mode that allows communication between an access point(AP) and a general node but does not allow direct communication betweennodes. The other is an ad-hoc mode that allows nodes to communicate datawith each other without using a medium connected to a network backbonesuch as an access point (AP).

The above two modes use Medium Access Control (MAC) methods based on aCarrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme, inorder to avoid the data collision at a reception node that may occur awireless medium is shared.

The CSMA/CA scheme uses two carrier sensing modes: a physical carriersensing mode and a virtual carrier sensing mode.

For transmission of data from a node A to another node, the physicalcarrier sensing mode checks whether another transmission is performed ona medium before transmitting data from a network interface card (NIC) ofthe node A. Ready-To-Send (RTS) and Clear-To-Send (CTS) control packetsare exchanged to avoid data collision, thereby solving hidden problemsthat may occur in the network.

Such control packets are also used in the virtual carrier sensing mode.Upon receipt of such control packet, neighbor nodes detect theinhibition of network access for a predetermined time from a NetworkAllocation Vector (NAV) contained in the received control packet. Thisis a medium access control method using the virtual carrier sensingmode.

In general, every IEEE 802.11 network interface card uses the physicalcarrier sensing mode mandatorily and uses a control packet for collisionavoidance optionally.

Even though medium access control is performed, simultaneous mediumaccess may occur in a predetermined time point, which leads to datacollision. In this case, a node experiencing the data collision waitsfor a selected number of times within a predetermined range of times andthen accesses the medium for data transmission. Such a collisionresolution method uses a random backoff scheme.

If a predetermined range of times increase twice for every collision,this case is called Binary Exponential Backoff (BEB). In this case, thenumber of time slots in a contention window increases twice for everycollision from an initial contention window, and a predetermined numberof time slots are selected among them. Thus, a node waits for thecorresponding time and then accesses a medium. These processes arerepeated to solve the collision problem.

Meanwhile, a physical carrier sensing range may be relatively increasedin order to minimize a data collision. In this case,simultaneous-transmission nodes are spaced apart from each other. Thus,the power of interference between transmission nodes can be reduced andthe probability of the success of data transmission through each linkcan be increased. However, more intermediate nodes are required in arelay network arranged in linear topology. Thus, a carrier sensing rangemust be set to be suitable for the trade-off between the above advantageand disadvantage.

Meanwhile, a target SIR of a network interface card may affect datatransmission. If the target SIR is set to be high, a relatively largeamount of data can be transmission by one successful transmissionprocess, but the data collision probability may increase. Therefore, atarget SIR must be set to be suitable for the trade-off between theabove advantage and disadvantage.

DISCLOSURE Technical Problem

An embodiment of the present invention is directed to providing a methodof optimal data transmission for improving a data transmission rate in amulti-hop wireless network, which can maximize an end-to-end throughputby minimizing a data collision that may occur during data transmission.

Another embodiment of the present invention is directed to providing amethod of optimal data transmission for improving a data transmissionrate in a multi-hop wireless network, which can minimize a datacollision and maximize an end-to-end throughput by calculating a carriersensing range value for a node with variable transmission power and bycontrolling the transmission power according to the calculated carriersensing range value or by adaptively adjusting a carrier sensingthreshold value for a node with constant transmission power.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention. Also, it is obvious to thoseskilled in the art of the present invention that the objects andadvantages of the present invention can be realized by the means asclaimed and combinations thereof.

Technical Solution

In accordance with an aspect of the present invention, there is provideda method for optima data transmission for improving a data transmissionrate of a node with variable transmission power in a multi-hop wirelessnetwork, the method including the steps of: obtaining channel stateinformation about a current wireless channel of the node; calculating acarrier sensing range in the number of hops using the obtained channelstate information, a target signal-to-interference ratio, and acontention window size in order to minimize data collision; calculatingthe number of nodes attempting data transmission based on signalsreceived from neighbor nodes, the number of the nodes attempting datatransmission being the number of contention nodes; and settingtransmission power adaptively according to the calculated carriersensing range value and the contention node numbers and transmittingdata at the set transmission power.

In accordance with another aspect of the present invention, there isprovided a method for optima data transmission for improving a datatransmission rate of a node with constant transmission power in amulti-hop wireless network, the method including the steps of: obtainingchannel state information about a current wireless channel of the node;setting a carrier sensing threshold using the obtained channel stateinformation, a target signal-to-interference ratio, the constanttransmission power, and a contention window size in order to minimizedata collision; and comparing the carrier sensing threshold with thereception power of a signal received from a neighbor node, determiningwhether to transmit data according to the comparison results, andtransmitting data accordingly.

ADVANTAGEOUS EFFECTS

If mobile nodes (e.g., cars and vehicles) equipped with IEEE 802.11network interface card are arranged in linear topology, when a sourcenode is to transmit data through intermediate relay nodes to adestination node, the present invention adjusts the relative distance orthe carrier sensing threshold of the simultaneous-transmission nodes toadjust the strength of the interference power received by each receptionnode, thereby making it possible to maximize an end-to-end throughput.

The present invention adaptively calculates a carrier sensing rangevalue for a node with variable transmission power, thereby making itpossible to minimize a data collision and maximize an end-to-endthroughput. Also, the present invention adaptively adjusts a carriersensing threshold value for a node with constant transmission power,thereby making it possible to minimize a data collision and maximize anend-to-end throughput.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the distribution of simultaneoustransmission nodes in a multi-hop wireless network to which the presentinvention is applied.

FIG. 2 is a flowchart illustrating a method for optimal datatransmission from a node with variable transmission power on a multi-hopwireless network in accordance with an embodiment of the presentinvention.

FIG. 3 is a flowchart illustrating a method for optimal datatransmission from a node with constant transmission power on a multi-hopwireless network in accordance with an embodiment of the presentinvention.

FIG. 4 is a graph illustrating the relationship between a target SIR anda carrier sensing threshold.

FIG. 5 is a graph illustrating the relationship between a target SIR andan end-to-end throughput.

BEST MODE

The advantages, features and aspects of the invention will becomeapparent from the following description of the embodiments withreference to the accompanying drawings, which is set forth hereinafter.

Therefore, those skilled in the field of this art of the presentinvention can embody the technological concept and scope of theinvention easily. In addition, if it is considered that detaileddescription on a related art may obscure the points of the presentinvention, the detailed description will not be provided herein. Thepreferred embodiments of the present invention will be described indetail hereinafter with reference to the attached drawings.

FIG. 1 is a diagram illustrating the distribution of simultaneoustransmission nodes in a multi-hop wireless network to which the presentinvention is applied.

FIG. 1 is drawn on the assumption that nodes on the multi-hop wirelessnetwork are located at regular intervals at the vertexes and centers ofhexagons. That is, a reference numeral 10 denotes a linear multi-hopwireless network where nodes 11 are linearly distributed along a road. Areference symbol R denotes the radius of a hexagon and a referencesymbol D denotes the shortest simultaneous transmission distance.

In FIG. 1, small black dots 11 denote nodes that are distributed on theroad represented by a thick straight line. The node is mounted on amobile unit, uses a half-duplex scheme, and may have an omni-directionalantenna. Hereinafter, the term ‘node’ denotes a mobile unit mounted witha terminal.

In FIG. 1, circles 100 to 112 also denote nodes, which represent thedistribution of nodes capable of performing simultaneous transmissionwithout affecting data transmission therebetween. The nodes 100 to 112,which are spaced apart from each other by at least the distance D, arecapable of simultaneous transmission.

The above linear distribution of the nodes is merely illustrative, whichis merely to perform simulations (see FIGS. 4 and 5) with ease. Thepresent invention can also be applied even when the nodes arenonlinearly distributed.

The present invention provides methods for optimal data transmission ina multi-hop wireless network, which may be implemented in the followingtwo schemes.

One is a scheme applied to a node with ‘variable’ transmission power,which detects the optimal ‘carrier sensing range’ value (see Equation 1below), compares the detected value with an idle state Inter-ArrivalTime (IAT) of the current transmission medium (wireless section), andobtains the desired performance by adaptively adjusting the transmissionpower according to the comparison results (see FIG. 2). Also, thisscheme is distributive and is thus easy to apply to actual mobileenvironments.

Another is a scheme applied to a node with ‘constant’ transmissionpower, which obtains the desired performance by adjusting the ‘carriersensing threshold’ (see Equation 6 below) of a network interface card(see FIG. 3).

In general, a target SIR for a node is provided from an applicationlevel. If the node can determine a target SIR value randomly, it maycalculate a target SIR of the maximum performance by using givenparameters.

Hereinafter, the above schemes will be described in detail withreference to FIGS. 2 and 3.

FIG. 2 is a flowchart illustrating a method for optimal datatransmission from a node with variable transmission power on a multi-hopwireless network in accordance with an embodiment of the presentinvention, which illustrates a data transmission process performed byeach node with variable transmission power.

The optimal carrier sensing range n will be described first beforedescribing a data transmission method with reference to FIG. 2.

The optimal carrier sensing range n used for transmission power controlis calculated based on the following Equation 1.

$\begin{matrix}{n = {{C\left( {\alpha,W_{0}} \right)}\gamma^{\frac{1}{\alpha}}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where n denotes the optimal carrier sensing range, which is representedby “Number of Hops” as n=D/R. If n is an integer, simultaneoustransmission is performed at another node spaced apart by n hops. If nis not an integer, simultaneous transmission is performed at theposition spaced apart by (the smallest integer greater than n) (i.e.,(the integer portion of n)+1) hops. C(α,W₀) is the solution of afourth-order polynomial found from a path-loss exponent α and an initialcontention window size W₀. γ denotes a target Signal-to-InterferenceRatio (SIR).

Hereinafter, the way to find the solution C(αW₀) will be described indetail.

Assume that nodes (i.e., mobile units) on a road forms linear topologyand that the other nodes are distributed very densely, as illustrated inFIG. 1. In this case, when a node performs data transmission, the othernodes within the corresponding carrier sensing range cannot perform datatransmission. Thus, a plurality of nodes must be spaced apart from eachother by at least the distance D (i.e., at least n hops) so that theyare capable of simultaneous transmission. Also, because of thedistribution with a sufficiently high density, the relative positions ofnodes in a transmission mode have the same form as the positions ofco-channel base stations of the hexagonal cellular system.

Thus, an RX SIR γ,(P) of an RX node located at one vertex of a hexagon(e.g., a hexagon formed by nodes 101 to 106) in FIG. 1 can be calculatedbased on the following Equation 2. That is, Equation 2 represents an SIRγ,(P) of a signal received at a node i (i.e., RX node) 100 located atthe center of the hexagon formed by the nodes 101 to 106.

$\begin{matrix}{{\gamma_{i}(P)} = \frac{\frac{X_{i}}{R^{\alpha}}}{{\sum\limits_{j = 1}^{6}\frac{Y_{j}}{D^{\alpha}}} + {\sum\limits_{k = 7}^{12}\frac{Y_{k}}{\left( {\sqrt{3}D} \right)^{\alpha}}} + {\sum\limits_{l = 13}^{18}\frac{Y_{l}}{\left( {2D} \right)^{\alpha}}} + \ldots}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

where X_(i), Y_(j), Y_(k), Y_(l), . . . are random variables of anindependent and identical distribution with an average value of 1.

If the SIR γ,(P) of the RX signal calculated by Equation 2 is largerthan a predetermined target SIR γ, a wireless link is connectedsuccessfully. If not, i.e., if the SIR γ,(P) of the RX signal calculatedby Equation is not larger than a predetermined target SIR γ, atransmission failure occurs and a retransmission is performed after apredetermined time by the binary exponential random backoff of mediumaccess control. Herein, the probability of failure of one wireless link,i.e., the wireless link failure probability P_(c) can be calculatedbased on the following Equation 3.

$\begin{matrix}{P_{c} = {{\Pr \left\lbrack {{\gamma_{i}(P)} \leq \gamma} \right\rbrack} = {1 - {\frac{1}{2}{^{\frac{1}{2}{\gamma {({{{- 2}u} + {v^{2}\gamma}})}}} \cdot {{erfc}\left( \frac{{- u} + {v^{2}\gamma}}{\sqrt{2}v} \right)}}}}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

where if α=4, then

${u = {{7\left( \frac{R}{4} \right)^{4}} = {7\left( \frac{1}{n} \right)^{4}}}},{v = {{2.5\left( \frac{R}{D} \right)^{4}} = {2.5{\left( \frac{1}{n} \right)^{4}.}}}}$

Also, u,v are represented by the carrier sensing range n by using therelationship

$n = {\frac{D}{R}.}$

Also, γ denotes a target SIR.

According to the paper [B.-J. Kwak, N.-O. Song and L. E. Miller,Performance analysis of exponential backoff, IEEE/ACM Trans. Networking,Vol. 13, No. 2, pp. 343-355, 2005], if the collision probability P_(c)is given, the average time N(P_(c), W₀) delayed due to the system ofbinary exponential random backoff can be calculated based on thefollowing Equation 4.

$\begin{matrix}{{N\left( {P_{c},W_{0}} \right)} = {{\frac{1}{2}\left( {\frac{1}{1 - P_{c}} + \frac{W_{0}}{1 - {2P_{c}}}} \right)} - 1}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

where W₀ denotes the initial contention window size used in the binaryexponential random backoff system and P_(c) denotes the collisionprobability.

The average time Δ(γ,n) taken to transmit a packet from a source node toa destination node can be calculated based on the following Equation 5.

$\begin{matrix}{{\Delta \left( {\gamma,n} \right)} = {n \cdot \left\lbrack {{\frac{1}{2}\left( {\frac{1}{1 - P_{c}} + \frac{W_{0}}{1 - {2P_{c}}}} \right)} - 1} \right\rbrack \cdot t_{slot}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

where t_(slot), denotes the duration of a single slot used in the binaryexponential random backoff system.

Because the erfc portion in the wireless link failure probability P_(c)can be approximated to “2”, Equation 3 can be substituted by

$P_{c} = {1 - {^{\frac{1}{2}{\gamma {({{{- 2}u} + {v^{2}\gamma}})}}}.}}$

When

$P_{c} = {1 - ^{\frac{1}{2}{\gamma {({{{- 2}u} + {v^{2}\gamma}})}}}}$

is applied to Δ(65 n) of Equation 5, the average time Δ(γl ) taken totransmit a packet from a source node to a destination node is expressedas a function of γ,n,W₀.

If the target SIR γ is fixed, Δ(γ,n) becomes a concave function of thenumber n of reuse hops and thus there is the optimal hop number n thatminimizes a delay time. In order to calculate the optimal hop number n,the exponential item of P_(c) is set to a variable X and Δ(γ,n) isdifferentiated to fine a point of “0”. In this case, for the convenienceof calculation, an exponential function can be approximated using aTaylor series and up to a fifth-order polynomial equation can beobtained. Arithmetically, a fifth or higher order equation has nogeneral solution and thus cannot be expressed as the closed-formsolution. Instead, an iterative tracking scheme can be used withincreasing a numerical accuracy. When more terms are omitted in theTaylor series, a fourth or lower order equation can be obtained.However, the accuracy of the obtained solution decreases.

When an X value of a point differentiated to “0” is determined, it ischecked whether the determined value satisfies the condition of

$0 < P_{c} < {\frac{1}{2}.}$

By doing so, X* satisfying the above condition is obtained finally.Because of the assumption of

${\frac{1}{2}{\gamma \left( {{{- 2}u} + {v^{2}\gamma}} \right)}} = X^{*}$

in the intermediate calculation process, when this equation is solved,the optimal reuse hop number (i.e., the number of hops for the optimalcarrier sensing range) of

$n = {{C\left( {\alpha,W_{0}} \right)}\gamma^{\frac{1}{\alpha}}}$

(see Equation 1) can be obtained.

In brief, the carrier sensing range value n expressed in the hop number(see Equation 1) is the solution of the equation

${\frac{1}{2}{\gamma \left( {{{- 2}u} + {v^{2}\gamma}} \right)}} = {X^{*}.}$

As an example, for W₀=4, C(α,4) has values of 11.59, 3.6, 2.83, 2.03,1.79 as the path-loss exponent a has values of 2, 3, 4, 5, 6.

Using the target SIR γ and the constants calculated above, the carriersensing threshold T_(CS), can be calculated based on the followingEquation 6.

$\begin{matrix}{T_{CS} = {\frac{\Pr}{{C\left( {\alpha,W_{0}} \right)}^{\alpha}}\frac{1}{\gamma}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

where Pr denotes transmission power and the remaining factors are thesame as described above.

There is a case where a network interface card (i.e., a node) canprovide a plurality of target SIRs. In this case, the optimal target SIRcan be calculated using given parameters α,W₀,n, T_(CS) as follows:

Assume that there are m target SIRs γ of the network interface card andthey are γ_(n), {n=1, 2, . . . , m}. The γ_(n) values vary depending onmodulation schemes.

In this manner, one node provides a plurality of target SIRs andsimultaneous transmission is performed at the hop of the optimal carriersensing range obtained above for a specific γ_(n). In this case, whendata are transmitted at a data transmission rate supportable for thespecific γ_(n), the throughput satisfying the maximum data transmissionrate can be obtained. Because each γ_(n) value has no uniform relationwith a data transmission rate, the γ_(n) with the maximum throughput canbe obtained experimentally.

Even when the optimal carrier sensing range is determined using Equation1, because the carrier sensing range is determined through a centralizedcalculation process, the determined carrier sensing range isunreasonable to apply directly to actual nodes (i.e., terminals) ofdistributed environments. In order to solve this problem, the presentinvention provides a function for adjusting the transmission power fordata transmission.

Hereinafter, the method for optimal data transmission from the node withvariable transmission power will be described in detail with referenceto FIG. 2.

Referring to FIG. 2, in step S200, a node desiring to transmit data at apredetermined data transmission rate, which includes a source node and arelay node, receives a pilot signal transmitted periodically from aneighbor infrastructure and analyzes the received pilot signal, therebydetermining channel state information of the current wireless channel,i.e., a path-loss exponent α. For example, the node analyzes thereception power of a pilot signal received periodically from a neighborinfrastructure (e.g., devices located on a road and transmit a busy toneor a pilot signal), determines the current channel state (e.g., whethera line-of-site environment or an environment with many reflectiveobjects), and determines a path-loss exponent α as one of 2 through 6according to the status determination results.

In the above-described embodiment, a path-loss exponent α is obtained byanalyzing a pilot signal received periodically from a neighborinfrastructure. In an alternative embodiment, a path-loss exponent α isobtained by analyzing signals received from neighbor nodes.

In step S202, the node calculates a carrier sensing range n, which iscapable of minimizing data collision, according to Equation 1 using apath-loss exponent α, a target SIR γ, and an initial contention windowsize W₀. That is, assuming that W_(o) of

$n = {{C\left( {\alpha,W_{0}} \right)}\gamma^{\frac{1}{\alpha}}}$

(see Equation 1) is “4” (may be different in actuality), C(α,4) hasvalues of 11.59, 3.6, 2.83, 2.03, 1.79 as a path-loss exponent α hasvalues of 2, 3, 4, 5, 6. Thus, when W₀=4 and the path-loss exponent αdetermined in step S200 are substituted, the carrier sensing range nbecomes one of 11.59, 3.6, 2.83, 2.03, 1.79. Herein, the target SIR γ iscalculated and provided by an application program of the correspondingnode, which is set to be optimal based on a data transmission ratesupportable by a network interface card.

Thereafter, the node senses neighbor nodes and measures an idle stateInter-Arrival Time (IAT). That is, when there is data to be transmitted,each node checks the time of an idle state of a propagation environment(e.g., a wireless transmission medium) to detect a time interval betweenidle states.

In step S204, using the idle state IAT, the node calculates the numberof nodes attempting data transmission (hereinafter referred to ascontention node number) based on the following Equation 7. In step S206,the node compares the calculated contention node number with the carriersensing range n.

$\begin{matrix}{{{Number}\mspace{14mu} {of}\mspace{14mu} {Contention}\mspace{14mu} {Nodes}} = {\frac{IAT}{K} + 1}} & {{Eq}.\mspace{14mu} 7}\end{matrix}$

where K denotes a transmission slot time of a node on the multi-hopwireless network. If K and the idle state IAT both have the time unit ofseconds, the contention node number can be compared with the carriersensing range n.

If the carrier sensing range n is larger than the contention nodenumber, the node increases the previous transmission power by one leveland transmits data at the increased transmission power, in step S208.

If the carrier sensing range n is equal to the contention node number,the node maintains the previous transmission power and transmits data atthe maintained transmission power, in step S210.

If the carrier sensing range n is smaller than the contention nodenumber, the node reduces the previous transmission power by one leveland transmits data at the reduced transmission power, in step S212.

The above processes are repeated to transmit data at the optimaltransmission power.

FIG. 3 is a flowchart illustrating a method for optimal datatransmission from a node with constant transmission power on a multi-hopwireless network in accordance with an embodiment of the presentinvention.

In general, an IEEE 802.11 node, specifically a network interface cardof the node has a constant carrier sensing threshold. If the networkinterface card is improved to correct the carrier sensing threshold, theoptimal carrier sensing threshold can be determined using a suitablecalculation scheme. Also, if each node NIC sets a target SIR accordingto a supportable target data transmission rate, calculates an optimalcarrier sensing threshold using the target SIR, and determines whetherto transmit data according to the optimal carrier sensing threshold, themaximum throughput can be obtained.

Referring to FIG. 3, in step S300, a node desiring to transmit data at apredetermined data transmission rate, i.e., a node with constanttransmission power, receives a pilot signal transmitted periodicallyfrom a neighbor infrastructure and analyzes the received pilot signal,thereby determining channel state information of the current wirelesschannel, i.e., a path-loss exponent α. A detailed description of this isthe same as that in FIG. 2.

In step S302, the node calculates a carrier sensing threshold T_(CS),which is capable of minimizing data collision, based on Equation 6 usinga path-loss exponent α, a target SIR γ, a predetermined transmissionpower Pr, and an initial contention window size W₀. Herein, the targetSIR γ is calculated and provided by an application program of thecorresponding node, which is set to be optimal based on a datatransmission rate supportable by a network interface card.

Thereafter, the node senses neighbor nodes and measures an idle stateInter-Arrival Time (IAT). That is, when there is data to be transmitted,each node checks the time of an idle state of a propagation environment(e.g., a wireless transmission medium) to detect a time interval betweenidle states.

In step S304, the node compares the reception power of a signal receivedfrom a neighbor node with the carrier sensing threshold T_(CS).

If the signal reception power is smaller than the carrier sensingthreshold T_(CS), the node transmits data with the predeterminedtransmission power Pr, in step S306.

If the signal reception power is larger than or equal to the carriersensing threshold T_(CS), the node defers data transmission in step S308and returns to step S300. The reason for this is that when sensing thereception power higher than the carrier sensing threshold T_(CS),through a network interface card, the node knows that it should nottransmit data because another node is transmitting data through awireless medium (e.g., a wireless channel).

FIG. 4 is a graph illustrating the relationship between a target SIR anda carrier sensing threshold. FIG. 5 is a graph illustrating therelationship between a target SIR and an end-to-end throughput.

The simulation for the present invention is an experiment that arranges15 nodes linearly and detects the relationship between the target SIRand the optimal carrier sensing threshold.

The constant C (see Equation 1) may be different from the simulationresult. The reason for this is that all parameters of actual conditionsare not considered in the present simulation.

The present simulation disregards constants and focuses on detecting thedegree of the similarity of the equation structure to the simulation.The simulation exhibits the satisfactory similarity to actualconditions.

A threshold value for determination of the carrier sensing range ispresent in a network interface card of each node, and the carriersensing range may be considered as being reciprocal to the thresholdvalue. Thus, FIG. 4 shows that the carrier sensing range n and thetarget SIR γ have a relationship of about n=C(α,W₀)γ^(1/α). If theconstant C is considered as a random value, only the product forms canbe seen.

FIG. 5 shows end-to-end throughput values (i.e., network throughputvalues) depending on the target SIRs in the graph of FIG. 4. It can beseen from FIG. 5 that the maximum throughput “30” is obtained when thetarget SIR is 8 dB.

As described above, the technology of the present invention can berealized as a program and stored in a computer-readable recordingmedium, such as CD-ROM, RAM, ROM, floppy disk, hard disk andmagneto-optical disk. Since the process can be easily implemented bythose skilled in the art of the present invention, further descriptionwill not be provided herein.

The present application contains subject matter related to Korean PatentApplication Nos. 2006-0124027, and 2007-0107682, filed in the KoreanIntellectual Property Office on Dec. 7, 2006, and Oct. 25, 2007,respectively, the entire contents of which is incorporated herein byreference.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. A method for optimal data transmission for improving a data transmission rate of a node with variable transmission power in a multi-hop wireless network, the method comprising: obtaining channel state information about a current wireless channel of the node; calculating a carrier sensing range in the number of hops using the obtained channel state information, a target signal-to-interference ratio, and a contention window size in order to minimize data collision; calculating the number of nodes attempting data transmission based on signals received from neighbor nodes, the number of the nodes attempting data transmission; and setting a transmission power adaptively according to the calculated carrier sensing range value and the contention node numbers and transmitting data with the set of transmission power.
 2. The method of claim 1, wherein obtaining the channel state information includes obtaining a pass-loss exponent on a wireless channel by analyzing a pilot signal received periodically from a infrastructure of the multi-hop wireless network adjacent to the node.
 3. The method of claim 1, wherein obtaining the channel state information includes obtaining a pass-loss exponent on a wireless channel by analyzing signals received from the neighbor nodes.
 4. The method of claim 1, wherein the target signal-to-interference ratio is set based on a data transmission rate supportable by a network interface card of the node.
 5. The method of claim 1, wherein transmitting the data includes: comparing the calculated carrier sensing range value with the contention node number; transmitting data at transmission power higher than the previous transmission power if the carrier sensing range value is larger than the contention node number; transmitting data at transmission power equal to the previous transmission power if the carrier sensing range value is equal to the contention node number; and transmitting data at transmission power lower than the previous transmission power if the carrier sensing range value is smaller than the contention node number.
 6. The method of claim 5, wherein the contention node number is calculated by obtaining an idle state inter-arrival time from the signals received from the neighbor nodes and dividing the idle state inter-arrival time by a transmission slot time of the node on the multi-hop wireless network.
 7. A method for optima data transmission for improving a data transmission rate of a node with constant transmission power in a multi-hop wireless network, the method comprising: obtaining channel state information about a current wireless channel of the node; setting a carrier sensing threshold using the obtained channel state information, a target signal-to-interference ratio, the constant transmission power, and a contention window size in order to minimize data collision; and comparing the carrier sensing threshold with the reception power of a signal received from a neighbor node, determining whether to transmit data according to the comparison results, and transmitting data accordingly.
 8. The method of claim 7, wherein obtaining the channel state information includes obtaining a pass-loss exponent on a wireless channel by analyzing a pilot signal received periodically from a infrastructure of the multi-hop wireless network adjacent to the node.
 9. The method of claim 7, wherein obtaining the channel state information includes obtaining a pass-loss exponent on a wireless channel by analyzing signals received from the neighbor nodes.
 10. The method of claim 7, wherein the target signal-to-interference ratio is set based on a data transmission rate supportable by a network interface card of the node.
 11. The method of claim 7, wherein transmitting the data includes: comparing the carrier sensing threshold with the reception power of the signal received from the neighbor node; transmitting data at the constant transmission power if the reception power of the signal received from the neighbor node is smaller than the carrier sensing threshold; and returning to the channel state information obtaining step without performing data transmission if the reception power of the signal received from the neighbor node is larger than or equal to the carrier sensing threshold. 